Make MeasureInvalidated event work correctly

Fixes dotnet#15177
Fixes dotnet#20181

src/Controls/src/Core/LegacyLayouts/Layout.cs

@@ -319,6 +319,12 @@ protected virtual void InvalidateLayout()
 		/// It is suggested to still call the base method and modify its calculated results.</remarks>
 		protected abstract void LayoutChildren(double x, double y, double width, double height);
 
+		internal override void OnChildMeasureInvalidatedInternal(VisualElement child, InvalidationTrigger trigger)
+		{
+			// TODO: once we remove old Xamarin public signatures we can invoke `OnChildMeasureInvalidated(VisualElement, InvalidationTrigger)` directly
+			OnChildMeasureInvalidated(child, new InvalidationEventArgs(trigger));
+		}
+
 		/// <summary>
 		/// Invoked whenever a child of the layout has emitted <see cref="VisualElement.MeasureInvalidated" />.
 		/// Implement this method to add class handling for this event.
@@ -603,14 +609,10 @@ void OnInternalAdded(View view)
 			{
 				InvalidateLayout();
 			}
-
-			view.MeasureInvalidated += OnChildMeasureInvalidated;
 		}
 
 		void OnInternalRemoved(View view, int oldIndex)
 		{
-			view.MeasureInvalidated -= OnChildMeasureInvalidated;
-
 			OnChildRemoved(view, oldIndex);
 			if (ShouldInvalidateOnChildRemoved(view))
 			{

src/Controls/src/Core/Page/Page.cs

@@ -497,6 +497,12 @@ protected override void OnBindingContextChanged()
 			if (TitleView != null)
 				SetInheritedBindingContext(TitleView, BindingContext);
 		}
+
+		internal override void OnChildMeasureInvalidatedInternal(VisualElement child, InvalidationTrigger trigger)
+		{
+			// TODO: once we remove old Xamarin public signatures we can invoke `OnChildMeasureInvalidated(VisualElement, InvalidationTrigger)` directly
+			OnChildMeasureInvalidated(child, new InvalidationEventArgs(trigger));
+		}
 
 		/// <summary>
 		/// Indicates that the preferred size of a child <see cref="Element"/> has changed.
@@ -706,9 +712,6 @@ void InternalChildrenOnCollectionChanged(object sender, NotifyCollectionChangedE
 				for (var i = 0; i < e.OldItems.Count; i++)
 				{
 					var item = (Element)e.OldItems[i];
-					if (item is VisualElement visual)
-						visual.MeasureInvalidated -= OnChildMeasureInvalidated;
-
 					RemoveLogicalChild(item);
 				}
 			}
@@ -721,20 +724,21 @@ void InternalChildrenOnCollectionChanged(object sender, NotifyCollectionChangedE
 				{
 					int insertIndex = index;
 					if (insertIndex < 0)
+					{
 						insertIndex = InternalChildren.IndexOf(item);
+					}
 
-					if (item is VisualElement visual)
+					InsertLogicalChild(insertIndex, item);
+
+					if (item is VisualElement)
 					{
-						visual.MeasureInvalidated += OnChildMeasureInvalidated;
-
-						InsertLogicalChild(insertIndex, visual);
 						InvalidateMeasureInternal(InvalidationTrigger.MeasureChanged);
 					}
-					else
-						InsertLogicalChild(insertIndex, item);
-
+
 					if (index >= 0)
+					{
 						index++;
+					}
 				}
 			}
 		}

src/Controls/src/Core/VisualElement/VisualElement.cs

@@ -1252,9 +1252,7 @@ protected override void OnChildAdded(Element child)
 		{
 			base.OnChildAdded(child);
 
-			var view = child as View;
-
-			if (view != null)
+			if (child is View view)
 			{
 				ComputeConstraintForView(view);
 			}
@@ -1377,6 +1375,35 @@ internal virtual void InvalidateMeasureInternal(InvalidationTrigger trigger)
 			}
 
 			MeasureInvalidated?.Invoke(this, new InvalidationEventArgs(trigger));
+			(Parent as VisualElement)?.OnChildMeasureInvalidatedInternal(this, trigger);
+		}
+
+		internal virtual void OnChildMeasureInvalidatedInternal(VisualElement child, InvalidationTrigger trigger)
+		{
+			switch (trigger)
+			{
+				case InvalidationTrigger.VerticalOptionsChanged:
+				case InvalidationTrigger.HorizontalOptionsChanged:
+					// When a child changes its HorizontalOptions or VerticalOptions
+					// the size of the parent won't change, so we don't have to invalidate the measure
+					return;
+				case InvalidationTrigger.RendererReady:
+				// Undefined happens in many cases, including when `IsVisible` changes
+				case InvalidationTrigger.Undefined:
+					MeasureInvalidated?.Invoke(this, new InvalidationEventArgs(trigger));
+					(Parent as VisualElement)?.OnChildMeasureInvalidatedInternal(this, trigger);
+					return;
+				default:
+					// When visibility changes `InvalidationTrigger.Undefined` is used,
+					// so here we're sure that visibility didn't change
+					if (child.IsVisible)
+					{
+						// We need to invalidate measures only if child is actually visible
+						MeasureInvalidated?.Invoke(this, new InvalidationEventArgs(InvalidationTrigger.MeasureChanged));
+						(Parent as VisualElement)?.OnChildMeasureInvalidatedInternal(this, InvalidationTrigger.MeasureChanged);
+					}
+					return;
+			}
 		}
 
 		/// <inheritdoc/>

src/Controls/tests/TestCases.HostApp/Elements/CollectionView/CollectionViewCoreGalleryPage.cs

@@ -47,6 +47,7 @@ public CollectionViewCoreGalleryContentPage()
 						// SelectionShouldUpdateBinding (src\Compatibility\ControlGallery\src\Issues.Shared\CollectionViewBoundSingleSelection.cs)
 						// ItemsFromViewModelShouldBeSelected (src\Compatibility\ControlGallery\src\Issues.Shared\CollectionViewBoundMultiSelection.cs)
 						TestBuilder.NavButton("Selection Galleries", () => new SelectionGallery(), Navigation),
+						TestBuilder.NavButton("Item Size Galleries", () => new ItemsSizeGallery(), Navigation),
 					}
 					}
 				};